I. Field of the Invention
The present invention relates to techniques for semiconductor processing, and more particularly to semiconductor processing which may be performed at low temperatures.
II. Description of the Related Art
In the field of semiconductor processing, there have been several attempts to use lasers to convert thin amorphous silicon films into polycrystalline films. For example, in James Im et al., xe2x80x9cCrystalline Si Films for Integrated Active-Matrix Liquid-Crystal Displays,xe2x80x9d 11 MRS Bullitin 39 (1996), an overview of conventional excimer laser annealing technology is presented. In such a system, an excumer laser beam is shaped into a long beam which is typically up to 30 cm long and 500 micrometers or greater in width. The shaped beam is scanned over a sample of amorphous silicon to facilitate melting thereof and the formation of polycrystalline silicon upon resolidification of the sample.
The use of conventional excimer laser annealing technology to generate polycrystalline silicon is problematic for several reasons. First, the polycrystalline silicon generated in the process is typically small grained, of a random microstructure, and having a nonuniform grain sizes, therefore resulting in poor and nonuniform devices and accordingly, low manufacturing yield. Second, in order to obtain acceptable performance levels, the manufacturing throughput for producing polycrystalline silicon must be kept low. Also, the process generally requires a controlled atmosphere and preheating of the amorphous silicon sample, which leads to a reduction in throughput rates. Accordingly, there exists a need in the field to generate higher quality polycrystalline silicon at greater throughput rates. There likewise exists a need for manufacturing techniques which generate larger and more uniformly microstructured polycrystalline silicon thin films to be used in the fabrication of higher quality devices, such as flat panel displays.
An object of the present invention is to provide techniques for producing uniform large-grained and grain boundary location controlled polycrystalline thin film semiconductors using the sequential lateral solidification process.
A further object of the present invention is to form large-grained and grain boundary location manipulated polycrystalline silicon over substantially the entire semiconductor sample.
Yet another object of the present invention is to provide techniques for the fabrication of semiconductors devices useful for fabricating displays and other products where the predominant orientation of the semiconductor grain boundaries may be controllably aligned or misaligned with respect to the current flow direction of the device.
In order to achieve these objectives as well as others that will become apparent with reference to the following specification, the present invention provides methods for processing an amorphous silicon thin film sample into a polycrystalline silicon thin film are disclosed. In one preferred arrangement, a method includes the steps of generating a sequence of excimer laser pulses, controllably modulating each excimer laser pulse in the sequence to a predetermined fluence, homoginizing each modulated laser pulse in the sequence in a predetermined plane, masking portions of each homogenized fluence controlled laser pulse in the sequence with a two dimensional pattern of slits to generate a sequence of fluence controlled pulses of line patterned beamlets, each slit in the pattern of slits being sufficiently narrow to prevent inducement of significant nucleation in region of a silicon thin film sample irradiated by a beamlet corresponding to the slit, irradiating an amorphous silicon thin film sample with the sequence of fluence controlled slit patterned beamlets to effect melting of portions thereof corresponding to each fluence controlled patterned beamlet pulse in the sequence of pulses of patterned beamlets, and controllably sequentially translating a relative position of the sample with respect to each of the fluence controlled pulse of slit patterned beamlets to thereby process the amorphous silicon thin film sample into a single or polycrystalline silicon thin film.
In a preferred arrangement, the masking step includes masking portions of each homogenized fluence controlled laser pulse in said sequence with a two dimensional pattern of substantially parallel straight slits spaced a predetermined distance apart and linearly extending parallel to one direction of said plane of homogenization to generate a sequence of fluence controlled pulses of slit patterned beamlets. Advantageously, the translating provides for controllably sequentially translating the relative position of the sample in a direction perpendicular to each of the fluence controlled pulse of slit patterned beamlets over substantially the predetermined slit spacing distance, to the to thereby process the amorphous silicon thin film sample into polycrystalline silicon thin film having long grained, directionally controlled crystals.
In an especially preferred arrangement, the masking step comprises masking portions of each homogenized fluence controlled laser pulse in the sequence with a two dimensional pattern of substantially parallel straight slits of a predetermined width, spaced a predetermined distance being less than the predetermined width apart, and linearly extending parallel to one direction of the plane of homogenization to generate a sequence of fluence controlled pulses of slit patterned beamlets. In this arrangement, translating step comprises translating by a distance less than the predetermined width the relative position of the sample in a direction perpendicular to each of the fluence controlled pulse of slit patterned beamlets, to the to thereby process the amorphous silicon thin film sample into polycrystalline silicon thin film having long grained, directionally controlled crystals using just two laser pulses. In one exemplary embodiment, the predetermined width is approximately 4 micrometers, the predetermined spacing distance is approximately 2 micrometers, and the translating distance is approximately 3 micrometers.
In an alternative preferred arrangement, the masking step comprises masking portions of each homogenized fluence controlled laser pulse in the sequence with a two dimensional pattern of substantially parallel straight slits spaced a predetermined distance apart and linearly extending at substantially 45 degree angle with respect to one direction of the plane of homogenization to generate a sequence of fluence controlled pulses of slit patterned beamlets. In this arrangement, the translating step provides for controllably sequentially translating the relative position of the sample in a direction parallel to the one direction of the plane of homogenization over substantially the predetermined slit distance, to thereby process the amorphous silicon thin film sample into polycrystalline silicon thin film having long grained, directionally controlled crystals that are disoriented with respect to the XY axis of the thin silicon film.
In yet another preferred arrangement, the masking step comprises masking portions of each homoginized fluence controlled laser pulse in the sequence with a two dimensional pattern of intersecting straight slits, a first group of straight slits being spaced a first predetermined apart and linearly extending at substantially 45 degree angle with respect to a first direction of the plane of homogenization, and a second group of straight slits being spaced a second predetermined distance apart and linearly extending at substantially 45 degree angle with respect to a second direction of the plane of homogenization and intersecting the first group at substantially a 90 degree angle, to generate a sequence of fluence controlled pulses of slit patterned beamlets. The corresponding translating step provides for controllably sequentially translating the relative position of the sample in a direction parallel to the first direction of the plane of homogenization over substantially the first predetermined slit spacing distance, to thereby process the amorphous silicon thin film sample into polycrystalline silicon thin film having large diamond shaped crystals.
In still another alternative arrangement, the masking step comprises masking portions of each homoginized fluence controlled laser pulse in the sequence with a two dimensional pattern of sawtooth shaped slits spaced a predetermined distance apart and extending generally parallel to one direction of the plane of homogenization to generate a sequence of fluence controlled pulses of slit patterned beamlets. In this arrangement, the translating step provides for controllably sequentially translating the relative position of the sample in a direction perpendicular to each of the fluence controlled pulse of slit patterned beamlets over substantially the predetermined slit spacing distance, to the to thereby process the amorphous silicon thin film sample into polycrystalline silicon thin film having large hexagonal crystals.
In a modified arrangement, an alternative technique for processing an amorphous silicon thin film sample into a polycrystalline silicon thin film using a polka-dot pattern is provided. The technique includes generating a sequence of excimer laser pulses, homoginizing each laser pulse in the sequence in a predetermined plane, masking portions of each homoginized laser pulse in the sequence with a two dimensional pattern of substantially opaque dots to generate a sequence of pulses of dot patterned beamlets, irradiating an amorphous silicon thin film sample with the sequence of dot patterned beamlets to effect melting of portions thereof corresponding to each dot patterned beamlet pulse in the sequence of pulses of patterned beamlets, and controllably sequentially translating the sample relative to each of the pulses of dot patterned beamlets by alternating a translation direction in two perpendicular axis and in a distance less than the super lateral grown distance for the sample, to thereby process the amorphous silicon thin film sample into a polycrystalline silicon thin film.
The accompanying drawings, which are incorporated and constitute part of this disclosure, illustrate a preferred embodiment of the invention and serve to explain the principles of the invention.